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  Sub-salt reflection tomography and imaging by walkaway vsp surveyUSPTO Application #: 20070259432Title: Sub-salt reflection tomography and imaging by walkaway vsp survey Abstract: A walkaway VSP survey is carried out with receivers located in a borehole near the base salt. Reflection tomographic inversion of data from the walkaway VSP is used to derive a velocity model for the subsurface and may be used for imaging of sub-salt reflections. (end of abstract) Agent: Madan, Mossman & Sriram, P.C. - Houston, TX, US Inventors: Xiaomin Zhao, Min Lou, James C. Jackson USPTO Applicaton #: 20070259432 - Class: 436025000 (USPTO) Related Patent Categories: Chemistry: Analytical And Immunological Testing, Geochemical, Geological, Or Geothermal Exploration The Patent Description & Claims data below is from USPTO Patent Application 20070259432. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCES TO RELATED APPLICATIONS [0001] This application claims priority from U.S. provisional patent application 60/797277 filed on 3 May 2006. The application is also related to an application being filed concurrently under attorney docket 504-43543-USD. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] This invention relates to a method of geophysical prospecting which improves the accuracy of seismic migration. Specifically, the invention uses a walkaway VSP survey for determination of subsurface velocities and imaging of reflections below salt layers in the earth. [0004] 2. Description of the Related Art [0005] In surface seismic exploration, energy imparted into the earth by a seismic source reflects from subsurface geophysical features and is recorded by a multiplicity of receivers. This process is repeated numerous times, using source and receiver configurations which may either form a line (2-D acquisition) or cover an area (3-D acquisition). The data which results is processed to produce an image of the reflector using a procedure known as migration. [0006] Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution and coherence due to the long and complicated travel paths between source, reflector, and receiver. Salt layers in the subsurface are particularly problematic. Due to the high compressional wave (P-wave) velocity of salt (4.48 km/s or 14,500 ft/s), there is considerable ray-bending of P-waves at the top and bottom of salt layers due to the large velocity contrast. Typical sedimentary velocities in the Gulf of Mexico may be no more than 3 km/s. [0007] Numerous approaches have been taken to address the problem of sub-salt imaging. These include using low frequencies, use of prestack depth migration, use of converted waves, redatuming to the base salt reflection, and seismic inversion. These have had limited success. [0008] The present invention uses a walkaway Vertical Seismic Profile (VSP) survey to estimate sub-salt velocities by tomographic inversion of reflection travel-times. In a walkaway VSP survey, measurements are made using a plurality of receivers in a borehole responsive to excitation of one or more seismic sources at a plurality of distances from the wellbore. The estimated velocities may then be used for migration of the walkaway VSP data or of surface seismic data. This method is particularly useful in the drilling of offset wells where an initial well that may or may not be productive has been drilled. Using the method of the present invention, it is possible to image the subsurface of the earth away from the initial well. SUMMARY OF THE INVENTION [0009] One embodiment of the invention is a method of estimating a property of an earth formation. The formation includes a first zone having a large impedance contrast with an overlying second zone. The method includes positioning a first sensor in the first zone, activating an energy source at a plurality of source positions at or near a surface of the earth, and recording signals from the first sensor responsive to the activation of the source. The recorded signals include a reflection from a lower surface of the first zone and an interface in the earth formation below the first zone. Travel-times are picked in the recorded signals corresponding to the reflections. The picked travel-times are used to estimate a position of the lower surface of the first zone, a position of the interface, and/or a velocity between the first zone and the interface. The first sensor may be positioned near the lower surface of the interface. The first zone may include salt. The picked travel-times may correspond to compressional wave travels-times. Estimating may be based on performing a tomographic inversion of the picked travel-times to give a velocity model. A wave-field separation of the recorded signal may be done. The method may further include identifying a zone of low-velocity. That method may further include migrating recorded signals using the velocity model. That method may further include performing drilling operations based on the velocity model. [0010] Another embodiment of the invention is a system for estimating a property of an earth formation including a first zone having a large impedance contrast with an overlying second zone. The system includes an energy source configured to be activated at a plurality of source positions at or near a surface of the earth. A first sensor is configured to produce signals responsive to the activation of the source, wherein the produced signals include a reflection from a lower surface of the first zone and an interface in the earth formation below the first zone. A processor is configured to pick travel-times from the recorded signals corresponding to the reflections, and estimate from the picked travel-times a position of the lower surface of the first zone, a position of the interface and/or a velocity between the first zone and the interface. The first sensor may be positioned near the lower surface of the first zone. The first zone may include salt. The processor may be further configured to pick travel-times corresponding to compressional waves. The processor may be configured to perform a tomographic inversion of the picked travel-times to give a velocity model. The processor may be further configured to identify a zone of low-velocity in the earth formation. The processor may be further configured to migrate the recorded signals using the velocity model. The system may further include a processor configured to use the output of the migration for further drilling operations. The first sensor may include a 3-C sensor and/or a hydrophone. The first sensor may be conveyed in a borehole using a wireline. [0011] Another embodiment of the invention is a computer-readable medium for use with a system for estimating a property of an earth formation including a first zone having a large impedance contrast with an overlying second zone. The system includes an energy source configured to be activated at a plurality of source positions at or near a surface of the earth. The system also includes a first sensor configured to produce signals responsive to the activation of the source, the produced signals including a reflection from a lower surface of the first zone, and an interface in the earth formation below the first zone. The medium includes instructions which enable a processor to pick travel-times in the recorded signals corresponding to the reflections, and estimate from the picked travel-times a position of the lower surface of the first zone, a position of the interface, and/or a velocity between the first zone and the interface. The medium may include a ROM, an EPROM, an EAROM, a flash memory, and an optical disk. BRIEF DESCRIPTION OF THE DRAWINGS [0012] The file of this patent contains at least one drawing executed in color: Copies of this patent with color drawing(s) will be provided by the Patent and Trademark Office upon request and payment of the necessary fee. The present invention is best understood by reference to the attached figures in which like numerals refer to like elements, and in which: [0013] FIG. 1 illustrates the geometry of data acquisition of a walkaway VSP according to the present invention; [0014] FIG. 2 is a display showing a velocity model used in simulation of a walkaway VSP; [0015] FIG. 3 shows an exemplary raypath geometry for the model of FIG. 2; [0016] FIG. 4 shows the insonfication of sub-salt reflections obtained for the model of FIG. 2; [0017] FIG. 5 is an exemplary simulated VSP data at a single depth for the model of FIG. 2; [0018] FIG. 6a shows an exemplary velocity model; [0019] FIG. 6b shows an initial model used for tomographic inversion; [0020] FIG. 6c shows the results of tomographic inversion of data simulated using the velocity model of FIG. 6a with an initial estimate shown in FIG. 6b; Continue reading... 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